Heat Transfer Background

Almost all food processes depend on or are affected by heat being added or removed at some stage in the operation. The efficient and effective utilization of heat results in economic savings, minimum adverse affects on nutrient components, higher quality consumer-ready products, and minimum effect on the many environmental factors associated with food processing. This efficient and effective use of heat depends on knowledge and subsequent application to install scraped-surface heat exchangers for both cooking and cooling duties when production requirements exceed 10,000 lb/day (Fig. 26). Not only does the SSHE provide faster cooking; it also improves product yield since moisture and fat loss is far less than the 15-20% decline in product volume that occurs with the open-kettle method (See Fig. 27.)

The typical meat cooking system consists of a single auger feed hopper with a leveling ribbon followed in line by a two-cylinder SSHE, one cylinder with 275°F steam and the other with 0°F ammonia. Both the auger feeder and SSHE are hydraulically driven from a central power source. After meat is ground, spiced, and blended, it is fed to the heat exchanger by means of a single-discharge auger designed specifically to move viscous, sticky products directly into a rotary pump. The auger maintains a constant stuffing pressure at the rotary pump inlet by varying the operating speed while a leveling ribbon maintains a uniform meat level in the feed hopper. The scraped surface blade and dasher design promote rapid removal of product from the cylinder walls and enhance agitation and mixing during both cooking and cooling phases. Assembly and disassembly for blade inspection and/or replacement is quick and easy. Meat remaining in the auger feed pump, pipelines, or the scraped surface heat exchanger is easily recovered and, once clear of product, the closed system and piping are easily cleaned with a pump circulation loop.

Products with Particulates

When processing sauces, gravies, or juices containing pieces of meat, vegetables, or fruit, one of the key elements is maintaining product identity by reducing or eliminating particulate breakage. This becomes more of a concern if the particulates exceed 1/4 in. in size and the carrier is a thin liquid such as broth or soup. Thick sauces, on the other hand, seem to have a cushioning effect with less product damage occurring during passage through the system.

While the dominant type of heat exchanger used for particulate-type products is the scraped-surface unit (Fig. 28), each case still must be considered individually. Many processes require multiple SSHE cylinders that should be piped in series rather than in parallel. This eliminates the need for individual product pumps while ensuring higher heat-transfer rates and an equal flow to each cylinder.

To reduce particle breakage, three design areas must be considered—the gap between the dasher and scraper blade, the sizing of SSHE inlets and outlets, and the dasher operating speed.

The distance between the underside of the blade and the dasher shaft (Fig. 29) should be equal or greater than the largest particle to be processed. This allows passage without particle damage as the blade sweeps by. The disadvantage of a wide annular space between the dasher and the cylinder wall is a longer product retention period and a reduction in exchanger efficiency. The faster the product travels through the cylinder, the higher the U value attained. This results in improved exchanger performance with lower residence time and less product damage.

The sizing of inlets and outlets is directly related to product quality. Using the largest possible ports reduces not only pressure drop through the system but also shearing effect. Furthermore, passage of product is eased and the possibility of contact with inlet walls is minimized.

Finally, there is the matter of dasher speed. As shown by the curve in Figure 30, while higher dasher speed results in a higher U value, it also can cause increased particulate damage. Therefore, higher dasher rpm and the corresponding high heat-transfer efficiency will apply only to products with small particulates. When products contain delicate particulates of 1/2-3/4 in., a variable-speed drive is used to reduce dasher revolutions to about 120 rpm. While this protects product quality, it also lowers exchanger efficiency. Consequently, striking a balance between performance and quality should be determined by lab runs so that optimum operation at full scale becomes a matter of a proper dasher speed setting.

Peanut Butter

For the production of creamy or chunky peanut butter, a process system will consist of surge tanks, a deaerator, scraped-surface heat exchanger, ingredient feeder with inline blender, and transfer pumps.

After mixing roasted nuts with a stabilizer, salt, and sugar to the desired formula, the product is ground and discharged at a temperature of about 150-200°F. Deaera-tion follows to eliminate air pockets, which initiate oil separation. During cooling from an SSHE inlet temperature of 140-190°F to a discharge temperature of 85-95° • F, the stabilizer is solidified in finely divided crystalline form and uniformly distributed throughout the mixture. At the proper temperature, the peanut butter becomes a viscous, extrudable mass. Crystal change continues and further solidification occurs after filling. For chunky style, a chunk feeder is located between the SSHE and filler or the transfer pump and deaerator.

Table 4 gives a listing of other products routinely processed using SSHEs.

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